Hitherto, as, for example, a roller bearing for supporting a spindle motor of hard disc devices, a roller bearing wherein the bearing ring and rolling element are formed with a ball bearing steel, etc., has become the main current.
In the above-described hard disc device, to prevent the occurrence of writing errors and reading errors, the improvement of the acoustic characteristics such as vibrations, noises, etc., has been required.
Also, because these information instruments are frequently placed near users at a relatively calm place in an office, a home, etc., it is strongly desired to reduce the generation of noises, particularly, having the frequency of the audible range of a human.
Under such circumstances, to quiet a rolling bearing for supporting spindle motor, it has been carried out to reduce the shape errors of each constituting element as small as possible, such as, for example, to improve the out of roundness of inner and outer rings and to equalize the sizes of all rolling elements used.
In addition, when the constituting elements of a rolling bearing are composed of a ball bearing steel, generally a heat treatment such as hardening, tempering, etc., is applied to the elements to control such that the residual austenite amount becomes about 10%.
Now, in the above-described conventional counterplan, there is a definite limit and thus further improvement has become necessary. Also, under the circumstance that the lubricating condition is severe, a rolling bearing is liable to generate heat as well as fine impressions, cracks, peelings, etc., are liable to form on the rolling elements and the bearing rings, whereby the caustic characteristics such as vibrations, noises, etc., are reduced.
Also, as the result of investigations, it has been confirmed that one of the causes of generating noises of a rolling bearing is the fine scratches formed on, in particular, rolling elements. On the other hand, recently, there are rolling elements formed with a ceramic material which is hard to be scratched. When the rolling element is formed with a ceramic material, the generation of noises by scratches can be restrained as well as because the material of the rolling element differs from the material of a bearing ring (steel-made), the effect of preventing sticking by coagulation of them by the inferior lubricant can be obtained.
As described above, the formation of rolling elements with a ceramic material is effective for improving the acoustic characteristics of a ball bearing but by the inventor's further investigations. It has been found that there remains the following point to be solved.
That is, when a rolling element is formed with a ceramic, because the rigidity of the rolling element is increased and the rolling element is hard to deform, the contact area of the rolling element and the bearing ring becomes small and the stress applied to the orbital surface becomes large. Accordingly, when the bearing ring is formed with a conventional ball bearing steel wherein the residual austenite amount is about 10%, the contact portion with the bearing element is liable to cause a permanent deformation and impressions form to generate vibrations and noises to deteriorate the caustic characteristics. Also, when a stress applied to the orbital surface becomes large, there is a problem that austenite is liable to cause a martensite transformation. Furthermore, in the case of using a ball bearing steel wherein the residual austenite amount is about 10%, there is a problem that the amount of the generated heat by the martensite transformation caused at raising the bearing temperature becomes large.